ICs generally include an isolation structure for electrically isolating devices from one another. Isolation options include trench isolation (e.g., Shallow Trench Isolation (STI)) or Local Oxidation of Silicon (LOCOS).
For LOCOS processing a thin silicon oxide layer is thermally grown on the wafer surface, generally called a pad oxide. A layer of silicon nitride is then deposited which is used as an oxide barrier. The pattern transfer is performed by photolithography. A lithography pattern is then etched into the silicon nitride. The result is a silicon nitride hard mask, which has openings that define the active areas defined by the LOCOS process.
The next step is the growth of the thermal LOCOS oxide which is termed a field oxide (FOX) as it is selectively grown in all non-active regions. After this LOCOS process is finished, the last step is the removal of the silicon nitride layer and the pad oxide layer underneath. A drawback of the LOCOS technique is the so-called bird's beak effect and the surface area which is lost due to this encroachment. Advantages of LOCOS fabrication include a simple process flow and the high FOX layer quality because the entire LOCOS structure is thermally grown.
STI is generally a preferred isolation technique for the sub-0.5 μm IC technology, because it avoids the bird's beak shape characteristic. With its zero oxide field encroachment STI is more suitable for the increased IC density requirements, as it enables forming smaller area isolation regions.